Process for recovering metals from resins

ABSTRACT

A process for recovering metals involving ion exchange comprising the step of recovering metal species from an ion exchange resin by elution of the resin with an eluent system containing (i) a first sulphite component comprising at least one of sulphite and bisulphite ion; and (ii) a second eluting component comprising any species which favours the ion exchange or desorption of a metal species from the resin during elution wherein the presence of the sulphite component (i) increases metal elution efficiency relative to the situation where an eluent comprises the second eluting component alone.

This invention relates to a process for recovering metals involving ion exchange.

Gold is usually recovered using a cyanidation leach process which involves leaching followed by recovery from solution using activated carbon. Thiosulphate leaching is a potential environmentally acceptable alternative to cyanidation and, in this process, the gold is leached as the gold thiosulphate complex. However, this complex is not readily adsorbed by activated carbon and so anion exchange resins may be preferred.

Gold may be loaded onto resins from either a slurry or a solution, but then the gold must be recovered from the resin by elution or desorption with organic or inorganic eluents or eluent systems. Gold can be eluted from resins using eluents such as thiocyanate, polythionate or nitrate based eluents. However, relatively concentrated solutions are required for the elution process. For example, in a nitrate elution process, 2 M ammonium nitrate is preferred as disclosed in PCT Application No. WO 01/23626 (Murdoch University). This is a relatively high concentration of nitrate that creates demonstrable cost implications for the elution step.

Thiocyanate solutions are known to rapidly elute gold (either cyanide or thiosulphate complexes) from resins. However, the resin must be regenerated prior to addition back into the resin in pulp circuit, otherwise the thiocyanate will accumulate in process water, eventually leading to environmental problems and reduced gold loading. In addition, the loss of thiocyanate may be economically unacceptable. Regeneration in the thiocyanate system is also complicated as thiocyanate is removed using ferric sulphate followed by regeneration of thiocyanate by addition of sodium hydroxide. This may lead to resin breakage from osmotic shock due to the swing in pH from elution to regeneration. A number of chemical reagents are also required at a plant site that may be remote. It is therefore desirable, subject to plant operational efficiency, to reduce the inventory of different chemicals used in plant operation. An aim is to use fewer reagents in lesser quantity.

A polythionate eluent system utilises a mixture of trithionate and tetrathionate. Since these species are strongly adsorbed on a resin, they can be used to effectively elute gold. However, the resin requires regeneration due to the high affinity of the polythionates for the resin. Regeneration is accomplished by treating the resin with sulphide ions to convert the polythionates to thiosulphate. A problem with polythionate elution is the stability of the solution. Tetrathionate undergoes a decomposition reaction to form trithionate and elemental sulphur, and in the presence of silver or copper, decomposes to precipitate copper or silver sulphides. Trithionate decomposes to form sulphate and sulphur. Such decomposition reactions result in losses that add to the cost of the process.

It is an object of the present invention to provide a process for recovery of metals by ion exchange which increases elution efficiency over conventional eluents with desirably lesser cost in terms of reagents, regeneration steps, inventory costs and the like.

With this object in view, the present invention provides a process for recovering metals involving ion exchange comprising the step of recovering metal species from an ion exchange resin by elution of the resin with an eluent system containing (i) a first sulphite component comprising at least one of sulphite and bisulphite ion; and (ii) a second eluting component comprising any species, particularly an anionic species, which favours the ion exchange or desorption of a metal species from the resin during elution wherein the presence of the sulphite component (i) increases metal elution efficiency relative to the situation where an eluent comprises the second eluting component alone. Strong base ion exchange resins are useful resins for the practice of the invention.

The process is particularly applicable to the elution of gold (and other precious metals) and may also be applied to other metals including base metals such as copper. It may be applied as an adjunct to any leach or other hydrometallurgical process for the extraction of such metals, including resin-in-pulp processes or other ion exchange unit operations. The process may be particularly advantageously applied to leached metal recovery following a thiosulphate leach process.

In this aspect, there may be provided a process for recovering precious metals comprising the steps of:

-   -   (a) leaching a precious metal containing material with a         thiosulphate solution;     -   (b) recovering leached precious metals by ion exchange with an         ion exchange resin; and     -   (c) eluting the ion exchange resin with an eluent system         containing (i) a sulphite component including at least one of         sulphite and bisulphite ion in combination with (ii) a second         eluting component containing an ionic species selected from the         group consisting of halide, nitrate, polythionate and         thiocyanate ionic species.

The process may also be applied to ion exchange for metal recovery following other hydrometallurgical processes.

Sulphite assisted elution involves elution of the ion exchange resin with an eluent that contains sulphite or bisulphite ion available as metal salts such as alkali metal salts (Na, K, Li and so on); or as derived from sulphur dioxide gas or metabisulphite, or reaction of sulphite with acids, such as hydrochloric acid, or reaction of metabisulphite with bases, such as sodium hydroxide. Such ions are purposefully added to various eluent solutions including any species, such as at least one anionic species selected from the group consisting of halide such as chloride, nitrate, polythionate such as trithionate, and thiocyanate with significant observable increases in the efficiency of metal elution, which may be measured in terms of bed volumes of eluent required to achieve a required level of metal elution from the resin. For instance, addition of sulphite to a trithionate eluent may result in a very high efficiency of gold elution, not observed with trithionate or sulphite alone. Indeed, sulphite—on its own—is not typically an effective eluent. Therefore, addition of sulphite to eluents surprisingly enables use of lower concentration of reagents in eluent solutions and cost reductions in plant operation.

Bisulphite could be added to the group of anionic species identified above, for instance where it is not selected as sulphite component (i).

The advantageous effects of sulphite addition may, without wishing to be bound by any theory, result from influence on speciation of the metal to be eluted, sulphite forming, potentially on interaction with the second eluting component, a mixed complex metal species, such as a gold mixed thiosulphate-sulphite species in the case of a thiosulphate leach scheme, with less affinity for the resin.

The second eluting component may include a solution of a single compound which may dissociate to form cation(s) and only one anionic species selected from the group consisting of halide, nitrate, polythionate and thiocyanate, though other related or effective eluting anions may be selected.

Eluent systems favoured for use in accordance with the present invention include systems containing sulphite and chloride ions. A particularly low cost effective eluent system involves addition of sulphite and/or bisulphite ion to a sodium chloride solution, such as a brine solution. Water sources available to metal recovery plants, including precious metal recovery plants, are often saline. Thus, the chloride/sulphite eluent system is economically attractive. Other favoured systems include sulphite in combination with nitrate, for example in ammonium nitrate form. Ammonium nitrate is useful in mining and its use in elution as well provides economic advantages as a lower inventory of chemicals can be maintained. Eluent systems to be used in the process are preferably simple and contain a minimal number of components, thus aiding in reduction of reagent and inventory costs.

Advantageously, sulphite concentration in eluent is greater than 0.01 M and is preferably in the range 0.05-1 M, allowing lower concentrations for the second eluting component than required for competing eluents resulting in cost advantages through lower reagent consumption.

Eluent stability may be improved in the presence of sulphite ion. For example, when a loaded resin contains tetrathionate, an unstable species, sulphite converts it to the more stable trithionate avoiding metal sulphide precipitation. The reaction scheme is as follows:

S₄O₆ ²⁻+SO₃ ²⁻<- ->S₂O₃ ²⁻+S₃O₆ ²⁻  [1]

Addition of sulphite to a trithionate (or polythionate) eluent is also beneficial because it reduces the formation of tetrathionate.

Sulphite ion may be added to the eluent system in admixture with bisulphite ion. A bisulphite-sulphite mixture or hydrosulphite solution, as formed—for example—by reaction of sulphite and acid (for example hydrochloric acid), may itself be an effective eluent. In this aspect, the present invention provides an eluent system containing hydrosulphite or a combination of (i) sulphite ions; and (ii) bisulphite ions, whether alone or in combination with other ionic species. Bisulphite is the protonated form of sulphite and hence the distribution of bisulphite and sulphite in solution is dependent on solution pH. The preferred pH range is 4.5-8 under which conditions there is a mixture of sulphite and bisulphite. The concentration of bisulphite+sulphite contained in the eluent system should be at least 0.2 M and is preferably in the range 0.5-2 M.

A bisulphite solution may be prepared by dissolving salts of bisulphite, metabisulphite or sulphite in water and adjusting pH using acid or alkali as necessary. Multiple sources of sulphite and bisulphite are available. Alternatively, SO₂ may be dissolved in alkaline solutions. One of the major advantages of a bisulphite eluent compared to other eluents is that, following elution, the resin may not require a regeneration step. Gold leach solutions are typically alkaline and contain copper (II). When a resin loaded with bisulphite is returned to leach, bisulphite converts to sulphite and is oxidised by copper (II) hence removing it from the resin. No regeneration is required.

Further, as bisulphite is oxidised to sulphate, its accumulation in the process water circuit does not pose the same concern as would a nitrate elution system.

Regeneration may also be avoided where sulphite in combination with sodium chloride is used as an eluent. Sodium chloride is an inexpensive reagent and brines or saline water sources are abundant in many gold regions making its use advantageous.

The metal recovery process of the present invention may be more fully understood from the following description made with reference to the following figures in which:

FIG. 1 is a schematic diagram of a thiosulphate resin in pulp process.

FIG. 2 is an elution curve demonstrating elution of gold from an anion exchange resin by 2 M sodium chloride (E1) and 2 M sodium chloride with 0.1 M sodium sulphite (E2);

FIG. 3 is an elution curve demonstrating elution of gold from an anion exchange resin by 0.5 M ammonium nitrate (E3); and 0.5 M ammonium nitrate in admixture with 0.1 M sodium sulphite (E4);

FIG. 4 is an elution curve demonstrating elution of gold from an anion exchange resin by 1 M ammonium nitrate (E5); and 1 M ammonium nitrate in admixture with 0.1 M sodium sulphite (E6);

FIG. 5 is an elution curve demonstrating elution of gold from an anion exchange resin by 0.1 M trithionate (E7); and an admixture of 0.1 M trithionate with 0.1 M sodium sulphite (E8);

FIG. 6 is an elution curve demonstrating elution of gold from an anion exchange resin by 0.2 M trithionate (E9); and 0.2 M trithionate and 0.1 M sodium sulphite (E10).

FIG. 7 is an elution curve demonstrating elution of gold from an anion exchange resin by hydrosulphite, or sulphite and bisulphite, a mixture of 1 M sodium sulphite and HCl adjusted to pH 6 to form bisulphite (E11).

FIG. 8 is a comparative diagram providing elution curves for all sulphite containing eluents tested.

In a preferred embodiment of the invention, gold and other precious metals are recovered into solution at a metal recovery plant by a thiosulphate leaching process followed by ion exchange to recover gold thiosulfate complex present in pregnant leach liquor from the leach step as shown schematically in FIG. 1.

In the ion exchange step, a strong base anion exchange resin is used to adsorb the gold thiosulphate complex. There are a number of commercially available strong base ion exchange resins which have an affinity to gold and which are useful for the ion exchange process. The functional group of most strong base resins is quaternary ammonium, R4N+. Such a resin, in sulphate or chloride form, is a Purolite A500 resin, as supplied by The Purolite Company of Bala Cynwyd, Pa., which is employed in a preferred embodiment of the invention. Any other anion exchange resin may, however, be used to comparable effect.

Following loading or adsorption of gold thiosulphate complex onto the resin, the gold must be recovered by elution; that is, desorbed. In the preferred embodiment, gold, and other metal values, are eluted from the resin by an eluent system containing sulphite ion in a sulphite assisted elution process. More specifically, the eluent system contains (i) a first sulphite component comprising at least one of sulphite and bisulphite ion; and (ii) a second eluting component comprising an anionic species which favours the ion exchange or desorption of a metal species from the resin during elution, the presence of the sulphite component (i) increasing the metal elution efficiency of the eluent relative to the situation where an eluent contains second eluting component alone. Various eluents, taking the form of aqueous solutions, may be used for this purpose.

For testing elution efficiency with various eluents, Purolite A500 resin was lightly packed into a glass column with a volume of 8 mL. Resin was loaded with gold thiosulphate, loading being achieved by shaking 10 g of clean resin in a 250 mL solution containing 250 mg/L Au, 0.1 M ammonium thiosulphate and 0.1 M ammonia overnight with low oxygen transfer.

The eluents and eluent systems tested, non-exhaustively and by way of illustration, were as follows:

E1 2 M sodium chloride

E2 2 M sodium chloride and 0.1 M sodium sulphite

E3 0.5 M ammonium nitrate

E4 0.5 M ammonium nitrate and 0.1 M sodium sulphite

E5 1 M ammonium nitrate

E6 1 M ammonium nitrate and 0.1 M sodium sulphite

E7 0.1 M trithionate

E8 0.1 M trithonate and 0.1 M sodium sulphite

E9 0.2 M trithionate

E10 0.2 M trithionate and 0.1 M sodium sulphite

E11 hydrosulphite, or sulphite and bisulphite, a mixture of 1 M sodium sulphite and HCl (pH 6)

In each case, sodium sulphite was used as a convenient source of sulphite ion. Other suitable metal sulphites are available and sulphur dioxide or metabisulphite may also be used as a source of sulphite. It will also be noted that each eluent system comprised at most two eluent components, this reducing inventory of chemicals required at the metal recovery plant.

A volume of 200 mL of each eluent or eluent system was pumped through the glass column at a speed of 5 bed volumes per hour (0.66 mL/min) with a fractional collector collecting 4 mL samples (0.5 bed volumes). 50 samples or 25 bed volumes were collected for each experiment. Samples were then diluted 20 fold with 0.01 M NaCN and 0.05 M Na₂CO₃ before being analysed by atomic absorption spectroscopy. The results were plotted as the elution curves of FIGS. 2 to 8.

FIG. 2 shows elution performance for the first pair of eluents E1and E2. Greater than 95% of gold was eluted from the resin after 13 to 14 bed volumes of 2 M NaCl/0.1 M sodium sulphite eluent pumped through the column. As NaCl is a very inexpensive reagent, being commonly available in brines and saline water in gold mining regions, the elution efficiency achieved through addition of sulphite ion is both technically and commercially significant.

FIG. 3 demonstrates elution performance for 0.5 M ammonium nitrate alone; and 0.5 M ammonium nitrate in combination with 0.1 M sodium sulphite (E3, E4). At a concentration of 0.5 M ammonium nitrate, 95% elution of gold could not be achieved under the test conditions yet, with 0.1 M sodium sulphite, greater than 95% gold elution was achieved by 20 bed volumes of eluent. A significant change in elution efficiency is therefore achieved by addition of sulphite ion. Without wishing to be bound by any theory, it is apparent that the addition of sulphite to a nitrate system allows formation of a species which has less affinity for the strong base anion exchange resin and which is significantly more easily eluted than the species present in the 0.5 M ammonium nitrate system.

In FIG. 4, showing performance for eluent pair E5, E6 the improvement resulting from increasing ammonium nitrate concentration to 1 M ammonium nitrate is seen. Still, much less than 95% gold elution is achieved even after 25 bed volumes of eluent have been pumped through the column. 0.1 M sodium sulphite addition, however, allowed greater than 95% gold elution by about 10 bed volumes of eluent, a significant improvement in performance. Comparative work for the 2 M ammonium nitrate eluent system provided in PCT Application No. WO 01/23626 (Murdoch University) suggests that more than 30 bed volumes for 2 M ammonium, sodium, potassium and nickel nitrate eluents will be required for greater than 95% gold recovery. Addition of sulphite ion, as taught by the present invention, therefore surprisingly offers substantially improved performance and economy when applied to nitrate systems because much lower nitrate concentration may be adopted (0.5 M) than in prior art nitrate systems.

FIG. 5 shows the improvement for 0.1 M trithionate elution (E7) when 0.1 M sodium sulphite is added. Further, the mixture of trithionate and sulphite (E8) is advantageous over a simple trithionate solution, as the sulphite converts any tetrathionate loaded on the resin from leaching to trithionate by reaction scheme [1] above. This prevents the formation of sulfur on the resin by reaction scheme 2 during the sulfide regeneration step which is required following trithionate elution.

S₄O₆ ²⁻+S²⁻<- ->2S₂O₃ ²⁻+S   [2]

FIG. 6 is also directed to the trithionate/sulphite eluent system, except that, here, the concentration of trithionate is raised to 0.2 M in eluent E9 with 0.1 M sodium sulphite being present in eluent E10. Sulphite addition is observed to further enhance eluent performance, or increased metal elution efficiency as measured by a reduced number of bed volumes to achieve, for example, 95% metal elution over the result where trithionate concentration is simply increased. That is, acceptable to very high elution efficiency may be achieved by presence of an effective amount of sulphite component (i) independently of increase in concentration of the second eluting component. This may have implications in terms of reducing trithionate usage and consequential costs for a given elution efficiency.

FIG. 7 shows elution performance for the hydrosulphite (1 M sodium sulphite and HCl (pH 6) eluent system E11, containing both sulphite and bisulphite ions, and showing greater than 95% elution of gold after 23 to 24 bed volumes of eluent have been pumped through the column.

Finally, FIG. 8 shows comparative elution performance for all the sulphite assisted eluent systems tested. The effect of sulphite ion on elution performance in the nitrate system may be particularly noted. However, good elution efficiency is observed for all the sulphite assisted eluents tested.

Modifications and variations to the metal recovery process of the invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed to be within the scope of the present invention. 

1. A process for recovering metals involving ion exchange comprising the step of recovering metal species from an ion exchange resin by elution of the resin with an eluent system containing (i) a first sulphite component comprising at least one of sulphite and bisulphite ion; and (ii) a second eluting component comprising any species which favours the ion exchange or desorption of a metal species from the resin during elution wherein the presence of the sulphite component (i) increases metal elution efficiency relative to the situation where an eluent comprises the second eluting component alone.
 2. The process of claim 1 wherein the second eluting component comprises at least one ionic species selected from the group consisting of halide, nitrate, polythionate and thiocyanate anionic species.
 3. The process of claim 2 wherein the second eluting component of the eluent system comprises one ionic species selected from halide, nitrate, polythionate, and thiocyanate anionic species.
 4. The process of claim 2 wherein the eluent system contains sulphite and chloride ions.
 5. The process of claim 4 wherein the eluent system contains sodium sulphite and sodium chloride.
 6. The process of claim 2 wherein the eluent system contains sulphite and nitrate ions.
 7. The process of claim 6 wherein the eluent system contains sodium sulphite and ammonium nitrate.
 8. The process of claim 1 wherein sulphite concentration in the eluent system is greater than 0.01 M.
 9. The process of claim 8 wherein sulphite concentration in the eluent system is in the range 0.05 to 1 M.
 10. The process of claim 1 wherein acceptable elution efficiency is achieved by presence of an effective amount of the sulphite component (i) independently of increase of concentration of the second eluting component.
 11. The process of claim 2 wherein the eluent system contains both sulphite and bisulphite ions.
 12. The process of claim 11 wherein pH of the eluent system is maintained within the range 4.5 to
 8. 13. The process of claim 11 wherein the combined concentration of sulphite and bisulphite ion in the eluent system is at least 0.2 M.
 14. The process of claim 13 wherein the combined concentration of sulphite and bisulphite ions in the eluent system is in the range 0.5 to 2 M.
 15. The process of claim 1 wherein the ion exchange resin may be used following an elution operation without regeneration.
 16. The process of claim 2 wherein the polythionate is trithionate, presence of sulphite reducing the formation of tetrathionate.
 17. A process for recovering precious metals comprising the steps of: (a) leaching a precious metal containing material with a thiosulphate solution; (b) recovering leached precious metals by ion exchange with an ion exchange resin; and (c) eluting the ion exchange resin with an eluent system containing (i) a sulphite component including at least one of sulphite and bisulphite ion in combination with (ii) a second eluting component containing an ionic species selected from the group consisting of halide, nitrate, polythionate and thiocyanate ionic species.
 18. The process of claim 17 wherein the concentration of sulphite component (i) in the eluent system is greater than 0.01 M.
 19. The process of claim 18 wherein the combined concentration of sulphite component (i) in the eluent system is in the range of 0.05 to 1 M.
 20. The process of claim 18 wherein sulphite component (i) includes sulphite and bisulphite ion and the combined concentration of sulphite and bisulphite ion in the eluent system is at least 0.2 M.
 21. The process of claim 20 wherein the combined concentration of sulphite and bisulphite ion in the eluent system is in the range of 0.5 to 2 M.
 22. The process of claim 1 wherein the resin is an anion exchange resin, preferably a strong base anion exchange resin.
 23. A process for recovering metals involving ion exchange comprising the step of recovering metal species from an ion exchange resin by elution of the resin with an eluent system containing hydrosulphite or a combination of (i) sulphite ions and (ii) bisulphite ions. 